INVESTIGATION OF OXIDE LAYERS FORMED ON CROFER 22 APU STEEL

摘要

The "Crofer 22 APU" is a trade name of the steel developed for use in high temperature solid oxide fuel cells (SOFCs). It is used as an interconnector material between cells and serves as an electrical current collector. Such a material has to fulfil several requirements, e.g., low electrical resistance, stability in an atmosphere of reactive gases at elevated temperature, matching of thermal expansion coefficient value to other constituents and, last but not least, low manufacturing cost. The Crofer steel meets most of these requirements; however, there is still a need to improve its stability in an oxidizing atmosphere. When exposed to working conditions, a layer of chromium oxide (Cr_2O_3) is formed on the steel surface. Chromia has a very good electrical conductivity, compared to Al_2O_3 and SiO_2, but can create volatile species that lower the efficiency of some SOFC components. Manganese is added to the steel to form chromium-manganese spinel on the oxide surface preventing chromium evaporation. Oxide layers grown on the Crofer surface at 800 ℃ in air during different times of exposure were chosen for investigations. The size and shape, as well as orientation of Cr_2O_3 and Mn_(1.5)Cr_(1.5)O_4 crystallites, were analyzed by means of 2D and 3D EBSD. The chemical composition of oxides was investigated using EDS and WDS to analyze the changes in concentration of steel additives like La, Cu Al and Si.